1. Field of the Invention
The present invention relates to an eleotrophotographic printing apparatus for thermally fixing a transferred image on paper.
2. Description of the Related Art
A laser printer is a well known electrophotographic printing apparatus. This type of printer selectively exposes a charged photosensitive body by means of a laser beam to form an electrostatic latent image thereon, deposits toner to develop the formed electrostatic latent image, and transfers the toner image onto a sheet of paper. After transfer, the paper is conveyed to a fixing unit, and the toner image is fixed on the paper by heat and pressure applied thereto while passing through heat and pressure rollers of the fixing unit.
The controller of the laser printer generally measures the fixing temperature by means of a temperature sensor, and controls the power supplied from a heater driver to a heater of the heat roller in accordance with the measured temperature to maintain the fixing temperature at a predetermined level. When overheating of the fixing unit is detected, the controller cuts off the power supply to the heater and stops conveyance of paper.
Overheating is sometimes occurs as a result of failure of a power transistor provided in the heater driver. If such a causes the power transistor to stay ON, the power supply to the heater cannot be cut off even when overheating is detected. If paper is passing through the fixing at the time overheating is detected, the paper will be intensely heated in the fixing unit and may catch fire. Even if the paper does not catch fire, it may be burned to a degree where it adheres to the heat rollers and the pressure rollers. In such a case, not only must the heater driver be repaired, but the burned paper must also be removed from the heat rollers and the pressure rollers in order that they can function normally, resulting in time-consuming and expensive repairs.